Garter carriage help needed, please

I have garter carriage KG-95 with a Brother 950i machine. I’m having difficulties getting started for the auto cast on. The Instruction Book says: “990 men button/men button insert 901 m” but the machine won’t accept 901 and the error light keeps coming up. This makes it a problem for the garter carriage to operate the cast on row. Also on the garter carriage when it’s operating, the slide No. 3 keeps changing and this changes the direction of the knitting. Can someone please help me? I live in Brisbane, Queensland, Australia and would appreciate any advice. Many thanks.